The present invention relates generally to active noise cancellation systems, and particularly to active noise cancellation systems for use in quieting the engine exhaust noise of an internal combustion engine. More particularly, this invention relates to an exhaust processor assembly including a mixing chamber receiving combustion product generated by a vehicle engine and a housing containing noise cancellation speakers for delivering cancelling sound waves to the mixing chamber.
Until recently, mufflers used to silence vehicle engines have been limited to passive devices. In a conventional muffler, a stream of exhaust gases is routed from the engine into the muffler where it is generally directed through various flow obstacles such as tubes and baffles provided inside the muffler. The purpose of the flow obstacles in a muffler is to reflect a portion of the sound waves associated with the exhaust gases back towards the engine. The change of acoustic impedance causes the sound energy to be reduced.
The use of such conventional baffle arrangements causes back pressure to develop in the exhaust system. As the stream of exhaust gases encounter the baffles or other obstacles to flow, a pressure wave is propagated back through the exhaust system to the engine, requiring additional power from the engine just to expel the exhaust gases. This power requirement results in reduced maximum power available from a given engine while decreasing fuel efficiency. Therefore, a muffler that is configured to eliminate or substantially reduce back pressure in an exhaust system and includes an active noise attenuation device would represent a great improvement over conventional mufflers.
Active noise attenuation devices use acoustic sensors, control systems, and speakers or transducers to produce sound wave interference. Using sound wave interference, undesirable noise is attenuated by mixing cancelling sound waves produced by the active noise attenuation device with the undesirable noise. The cancelling sound waves are ideally the same frequency and amplitude as the undesirable noise produced by the engine exhaust, but 180 degrees out of phase. When sound waves of equal amplitude and frequency but opposite phase interact, they cancel. In an engine exhaust system equipped with an active noise attenuator, the cancelling sound waves produced by the attenuator mix with the sound waves associated with the exhaust gases traveling in an engine exhaust system to quiet the engine exhaust noise to an acceptable level.
It has been observed that various components in an active noise attenuation device are susceptible to damage as a result of exposure to excessive heat. For example, speakers used to produce cancelling sound waves could be damaged by exposure to heat from the exhaust gases radiated by an exhaust pipe. This exposure to heat can cause the speakers to overheat and the adhesive holding the speaker cones to the speaker frames to deteriorate, thereby allowing the speaker cones to separate from the speaker frame. Consequently, the effectiveness of the speakers in a hot active noise attenuation device could be reduced or completely compromised.
An improved active noise attenuation device would be thermally isolated from contact with the high heat of the combustion product produced by an engine and discharged through an exhaust system. Thermal isolation of the active noise attenuation device from hot combustion product would minimize heat damage to the device without sacrificing noise attenuation. Furthermore, the improved design could eliminate as many bends in the exhaust pipe as possible, along with removing the restrictive muffler, thereby reducing the back pressure applied to the engine.
According to the present invention, an exhaust processor assembly includes a housing formed to include an interior chamber and outlet means for emitting sound waves generated in the interior chamber. The assembly further includes means for providing an acoustical mixing chamber in acoustical communication with the outlet means, means for conducting combustion product from an engine to the acoustical mixing chamber, and means for producing sound waves to attenuate noise generated by combustion product introduced into the acoustical mixing chamber through the conducting means. The assembly further includes means for mounting the sound wave producing means in the interior chamber of the housing to partition the interior chamber to define a first sub-chamber receiving the sound waves generated by the sound wave producing means and having an opening communicating with the outlet means and a second sub-chamber providing a resonance chamber in spaced-apart relation from the outlet means.
In preferred embodiments, the mounting means includes a bracket coupled to the housing to partition the interior chamber of the housing into first and second sub-chambers. The sound producing means illustratively includes a pair of speakers mounted on the bracket and oriented to produce sound waves in the first sub-chamber. The sound waves produced in the first sub-chamber by the speakers are communicated to the acoustical mixing chamber through the outlet means. Illustratively, the two speakers are mounted on a V-shaped bracket so that they are aligned at an angle to one another and arranged to face toward the outlet means formed in the housing.
Advantageously, the hot noisy combustion product mixes with the noise attenuating sound waves produced by the speakers in the acoustical mixing chamber. This acoustical mixing chamber is located outside of the housing and away from the speakers (e.g., in the tail pipe). Heat damage to the speakers is minimized because the remote location of the speakers relative to the acoustical mixing chamber functions to protect the speakers from exposure to the hot noisy combustion product passing through the acoustical mixing chamber.
By providing an acoustical mixing chamber outside of the housing and in acoustical communication with the interior chamber and the conducting means, the improved exhaust processor assembly of the present invention causes noise generated by the engine combustion product to be cancelled in the acoustical mixing chamber outside the housing by the sound waves produced by the speakers in the remote interior chamber inside the housing. Furthermore, the noise cancellation can take place without requiring the hot engine combustion product to pass through the interior chamber and come into contact with the speakers mounted therein, thereby minimizing heat and corrosion related problems found in conventional electronic mufflers. By thermally insulating the acoustical mixing chamber from the outlet means of the interior chamber using a ring made of insulation material, heat generated by the engine combustion is restricted to the acoustical mixing chamber and is not passed back into the interior chamber containing the speaker through the outlet means. This reduces the operating temperature in the speaker environment inside the interior chamber of the housing to yield increased durability and longevity of the speakers, thereby providing an improvement over conventional electronic mufflers.
In other preferred embodiments, the exhaust processor assembly includes an outer housing having an interior chamber and a first outlet for emitting low-frequency sound waves generated by low-frequency speakers positioned in the interior chamber. It also has an inner housing having a second interior chamber and second outlet for emitting high-frequency sound waves generated by high-frequency speakers positioned in the second interior chamber. The inner housing is situated inside the outer housing to position the second outlet of the inner housing inside the first outlet of the outer housing.
By providing an inner housing inside the outer housing, certain embodiments in accordance with the present invention function to allow the production of high-frequency and low-frequency sound waves in two separate but acoustically connected chambers, advantageously maximizing use of available space. By using an acoustical mixing chamber separate from the inner and outer chambers, noise cancellation can take place without requiring the hot engine combustion product to pass through either one of the inner and outer chambers, thereby minimizing heat and corrosion related problems found in conventional electronic mufflers. By thermally isolating the acoustical mixing chamber from the inner and outer chambers using one or more rings made of insulation material, heat generated by the engine combustion is restricted to the acoustical mixing chamber and is not passed back into the chambers containing the speakers through the first and second outlets.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.